The present invention relates to a device for placing small volumes of a material being in a liquid or fluid shape such as viscous media and liquid dispersions, in particular a viscous dispersion such as solder paste and different kinds of glues and adhesives, with a high speed on a substrate or base by ejecting drops of the material from a chamber through a nozzle.
When mounting electronic components on printed circuit boards and substrates it can be required that small isles or patches of solder paste are placed with a high speed and a high accuracy on such a base. Also, depositing varnishes and glues, in particular an electrically conductive adhesive, can be required. The high speed is required for the ever higher speed at which the electronic devices of today are produced.
In the published International patent application WO91/12106 a device is disclosed for depositing material, which comprises a rod, the length of which is changed when it is subjected to a suitable magnetic field. The end of the rod forms one wall of a chamber filled with the material. When the length of the rod is suddenly changed to become longer, the volume of the chamber is changed and the material is in the shape of a drop pressed out of a nozzle. In the published International patent application WO90/00852 instead the volume of the chamber is changed by making the chamber of a piezo-electrical material and subjecting it to an electric field.
In the European patent document EP-B1 0 517 767 a device is disclosed for depositing drops of a liquid material such as solder paste, glue and similar materials. The device comprises a rigid metal pipe through which material flows and which contains a nozzle in a wall of the pipe, and it further comprises a rod resting at the outside of the pipe conduit and made of an magnetostrictive material. By subjecting the rod to a magnetic field it will change its length to produce a blow on the outside of the pipe thereby locally increasing the pressure in the pipe, so that a drop of the material is ejected through the nozzle.
In U.S. Pat. No. 5,320,250 a method is disclosed for rapid dispensing of small amounts of a viscous material. A chamber containing the material has a nozzle in a wall and another wall of the chamber has the shape of a flexible, elastic diaphragm. An impact mechanism such as an electrically actuated hammer hits against the outside of the diaphragm in order to produce a change of the volume of the chamber, so that a drop is pressed out of the nozzle.
It is an object of the invention to provide a device by means of which small amounts of a liquid or fluid material can be ejected from a chamber containing the material, so that one drop at a time can be ejected in an accurately defined volume or so that the liquid material is ejected in a finely divided shape.
Thus, in a device for dispensing small quantities or amounts of a material a chamber is provided having a nozzle aperture arranged in a first wall. The chamber has a second wall which is opposite and parallel to the first wall. A rod is movably mounted in or attached to the second wall in such a way that one end surface of the rod and a neighbouring portion of the envelope surface of the rod are located inside the chamber. This end surface is furthermore opposite the nozzle aperture. An actuating device is coupled to this rod in order to displace it through a short distance forwards and backwards or otherwise produce, e.g. by changing the length of the rod. Then the one end surface of the rod moves forwards and backwards inside the chamber, in the longitudinal direction of the rod, per pendicularly to the one end surface. The diameter or largest cross dimension of the rod is advantageously large compared to the diameter of the nozzle aperture. The length of stroke of the rod or equivalently the displacement of the end surface of the rod is small but the stroke movement is made with a high acceleration and a large force, so that a pressure wave is thereby formed and propagates in the viscous medium. When hitting the inlet of the nozzle channel, the pressure wave then ejects material through the nozzle. The ejection of the material is thus primarily produced by a pressure wave or a pressure shock which propagates in the material and not by a change of the volume of the chamber. Furthermore, in order that the pressure wave will efficiently act towards the nozzle aperture the end surface of the rod is advantageously located close thereto. Thus, the distance between the end surface of the rod and the inlet of the nozzle aperture is preferably small compared to the diameter of the end surface. Said diameter can be taken as equivalent to the largest dimension of the end surface as taken in a cross direction of the rod, i.e. from an edge of the end surface to an opposite edge thereof.
The displacement of the rod or its end surface can easily be produced maintaining a high accuracy of the movement without having it degraded because of wear or plays in bearings and similar reasons. Examples of suitable driving devices are electrostrictive, piezo-electrical, magnetostrictive actuators and memory metal actuators.